This invention relates to active matrix electroluminescent display devices, comprising an array of electroluminescent display pixels arranged in rows and columns. The invention is particularly concerned with display devices in which rows of pixels share a common line, with currents through the display elements of the row passing along the common line.
Matrix display devices employing electroluminescent, light-emitting, display elements are well known. The display elements may comprise organic thin film electroluminescent elements, for example using polymer materials, or else light emitting diodes (LEDs) using traditional III-V semiconductor compounds. Recent developments in organic electroluminescent materials, particularly polymer materials, have demonstrated their ability to be used practically for video display devices. These materials typically comprise one or more layers of a semiconducting conjugated polymer sandwiched between a pair of electrodes, one of which is transparent and the other of which is of a material suitable for injecting holes or electrons into the polymer layer. An example of such is described in an article by D. Braun and A. J. Heeger in Applied Physics Letters 58 (18) p.p. 1982-1984 (May 6, 1991).
The polymer material can be fabricated using a CVD process, or simply by a spin coating technique using a solution of a soluble conjugated polymer.
Organic electroluminescent materials exhibit diode-like I-V properties, so that they are capable of providing both a display function and a switching function, and can therefore be used in passive type displays.
However, the invention is concerned with active matrix display devices, with each pixel comprising a display element and a switching device for controlling the current through the display elements. Examples of an active matrix electroluminescent display are described in EP-A-0653741 and U.S. Pat. No. 5,670,792, the contents of which are incorporated herein by way of reference material.
A problem with display devices of this type arises from the fact that they have current driven display elements. Display devices of the type with which this invention is concerned include a common line on which the currents from all pixels in a row pass. Compounding currents from the pixels in a row give rise to different voltages along the common line. These voltages depend upon the currents through all pixels in the row, since these currents all pass to the common line. These different voltages give rise to undesired changes to the outputs from the display pixels, which vary as a function of the full set of signals applied to the row. Consequently, there is cross-talk between the pixels within the row.
According to the invention, there is provided an active matrix electroluminescent display device comprising an array of display pixels arranged in rows and columns, each pixel comprising an electroluminescent display element and a switching means for controlling the current through the display element based on a signal voltage applied to the pixel, each row of pixels sharing a common line, currents through the display elements of a row of pixels passing along the common line, wherein the width of each common line tapers from one end to the other, the common line being coupled to row driver circuitry at the wider end.
By making the common lines wedge-shaped, the resistance of the common line can be reduced dramatically at the end of the common line near to the row driver circuitry. It is at this end of the common line that the current flowing is derived from all pixels within the row, and that voltage drops are therefore highest.
Preferably, the common lines of alternate rows taper in opposite directions, thereby defining an interleaved pattern of wedge shaped common lines. In this case, first and second row driver circuits may be provided at opposite ends of the rows of pixels.
Preferably, the common lines are arranged over a substrate, and the display pixels are arranged over the common lines, and wherein the display pixels emit light away from the substrate. The tapered shape of the common lines does not then result in any visual artifacts in the display image, and the pixels may be arranged as a regular array of equal size pixels.